Cast coating method



Nov. 8, 1966 V. A. LAUDERMAN ETAL CAST COATING METHOD Filed June 1, 1962II T T ORA/5y United States Patent Ofiice 3,284,251 Patented Nov. 8,1966 3,284,261 CAST CUATTNG METHOD Vincent A. Lauderman and Harry S.lledley, Hamilton, Ohio, assignors to Champion Papers llnc., Hamilton,Ohio, a corporation of Ohio Filed June 1, 1962, Ser. No. 199,315 4Claims. (Cl. 156-280) The present invention relates to the cast coatingof cellulosic paper webs and more particularly to a process involvingthe simultaneous lamination of separate paper webs and the cast coatingthereof.

The manufacture of cellulosic paper webs having a cast coating, i.e. onein which the surface is a replica of a finishing surface such as apolished chromium drum, is described in US. Patent 1,719,166. Theso-called cast coated products thus obtained have found widespread usein a variety of applications owing to their unique combination ofoutstanding high gloss finish and to the excellent resolution affordedby all types of conventional printing processes.

Although in fulfillment of the needs for these many applications, castcoated papers have been commercially manufactured in a broad range ofbasis weights, there has been particular difiiculty in economicallymanufacturing a suitable quality cast coated paper from the heavierbasis weight or higher caliper stocks. Such products, ideally employedfor a variety of packaging uses such as folding boxes and cartons, haveheretofore been inordinately expensive because of the comparatively lowoperating speeds associated with their manufacture. These speeds havepreviously been regarded as necessary with such heavy stocks in order tominimize the formation of curl and provide the characteristic of a fiatand level cast surface. Although a number of techniques have beensuggested, often satisfactorily employed, for increasing the speed ofthe cast coating process, in general these have not been whollysatisfactory for the manufacture of heavier basis weight cast coatedproducts. In this regard the technique of U.S. Patent 2,360,919 toWarner involving the application of water to the uncoated side of thesheet during the casting surface to increase speed and reduce curl isless well adapted to heavy weight stocks which are more slowlypenetrated by the applied Water,

For many applications, paper sheet materials with a cast surface havebeen obtained by merely laminating a light weight cast coated paper, forexample a litho grade, to a heavier basis weight sheet. This method hasnot, however, been fully satisfactory from the standpoint of either theeconomics, an extra converting operation being required, or the productacceptability, any irregularities in the heavier sheet being pressedinto the cast coated sheet with resultant losses in flatness.Furthermore a considerable amount of care has been required to preventthe adhesive vehicle from striking through the paper web and impairingthe finish of the cast surface.

In accordance with this invention, it has been found that the problemsheretofore encountered with the manufacture of heavy weight cast coatedstocks can be successfully overcome by a procedure involving thesimultaneous lamination of two separate plies of cellulosic paper webswhile providing one of the exposed surfaces of the plies with a castcoating.

More particularly, according to the invention cast coated stocks ofrelatively heavy basis weight are manufactured by the lamination of twomoving plies of paper by means of an intermediate layer of aqueous basedbonding composition containing a tacky adhesive. Before substantialevaporation of water contained in the bonding composi' tion, an exposedsurface of one of the plies is contacted with an aqueous based mixtureof paper coating adhesive and finely divided mineral pigment. Thelaminated and coated sheet material is dried against a finished castingsurface and finally stripped from that surface.

The foregoing process is particularly advantageous for the manufactureof cast coated stocks of a comparatively heavy basis weight, e.g thosehaving a caliper of 0.014 inch or more. For this purpose a number ofsignificant advantages can be obtained from the standpoint of productimprovement, process simplification and versatility, and economics.

Thus according to the invention, several advantages accrue from theregulation of the process such that a significant amount of water in thebonding composition is not evaporated prior to contacting the castingsurface. In the first place the water within the substrate serves tointernally plasticize and soften the combined webs such that a greaterdegree of conformity to the finishing surface is facilitated without theneed for excessive compression which would reduce the caliper. Suchconformity, and the resulting high degree of flatness and levelness ofthe product, would not be readily obtained in the absence of theplasticizing effect, whether the substrate was a single ply or doubleply.

Secondly, the presence of water within the substrate actually serves toassist in volatilizing the water of the coating composition while incontact with the casting finishing surface such that increases inoperating speed can be obtained. In this regard, as well as from thestandpoint of reduced curl, the process accomplishes much the samebeneficial results as the technique of the aforementioned Warner patentwhich is less suitable for the manufacture of heavier basis weight castcoated stocks.

The process of the invention affords still other significant advantages.Thus it normally requires considerably less of the cast coatingcomposition to achieve a flat and level surface in comparison with thatwhich would be required with a substrate comprising a single ply ofgreater thickness, i.e. increases in surface irregularities of a web canbe expected as the caliper increases and these must be filled in to givea true cast surface. Even aside from the economic benefits therebyafforded, the resultant prodnot is thus characterized by a reducedtendency of the coating to fracture upon folding. As a further benefitof reducing the coating thickness, the tendency of the product to curl,normally toward the coated side in proportion to amount of coating, isdecreased.

Of significant practical value, the process of the invention affordswide versatility with respect to the utilization of available papermanufacturing equipment in order to provide a cast coated product of thedesired caliper. Thus a manufacturer not possessing a cylinder or otherpaper machine capable of producing the heavier basis weight stocks canconveniently laminate two lighter weight stocks without the need for aseparate operation, Indeed there would be no obstacle to the laminationand casting of three or more webs by the instant process although thecomplexities of the unwind system would ordinarily make thisunattractive for most purposes.

The invention will be further described with reference to the drawing.

Two moving webs of paper 18 and 23 are separately unwound from rolls 15and 20 respectively. Although it will be apparent that variousarrangements and modifications can be utilized to provide a layer ofaqueous bonding composition between the webs in parallel superimposedrelationship, the apparatus as illustrated employs coater 25 to applysuch a layer to web 18. For this purpose guide rolls 21 areappropriately positioned to adjust the angle of contact with the coaterand to thereby control the amount of bonding composition so applied. Thenature of coater 25 is not critical, rather it may be any of the wellknown roll coaters, knife coaters and the like. As shown, the coatersimply comprises a rotating roll 28 dipping in a trough 24 containingbonding composition 26, with a doctor rod 27 to smooth the layer ofbonding composition and remove any excess.

Following the application of bonding composition containing tackyadhesive to a surface of one, or both if desired, of the running Webs,the plies are at least superficially bonded to one another by a positivepressure exerted by the nip of rotating pressure rolls 35. Desirablythese rolls will have a resilient exterior layer, e.g. rubber, in orderto minimize compression of the sheets. As a result of the partialbonding, virtually no separation or slippage of the plies will beencountered throughout the remainder of the operation and hence anytendency toward blistering will be avoided.

After bonding of the plies in the foregoing manner, one of the twoexposed faces thereof is provided with an aqueous coating comprising amixture of paper coating adhesive and finely divided mineral pigment. Ina preferred embodiment this coating is the same as the bondingcomposition 26; that is, the paper coating adhesive serves as the tackylaminating adhesive. In this way a single coating supply can be employedfor both coaters and there is no problem if a small amount of the firstapplied coating is inadvertently carried over into the second coater. Inaddition, the tendency of the product to curl upon drying by virtue ofdiffering compositions is at least partially minimized.

As hereinbefore indicated it is essential that the water of the bondingcomposition not be volatilized from the laminate before being pressedinto contact with the casting finishing surface 55. Thus although thewater will be in part absorbed by the adjacent plies of paper, it shouldbe largely confined within the laminate in order to afford the operatingadvantages hereinbefore enumerated.

With regard to coater 45, this may be similar or different from coater25. As shown it comprises a turning roll 48, doctor rod 47 and asuitable pan or trough 49 containing coating mixture 46. Guide roll 41,being adjustable in a vertical position, serves to regulate the quantityof coating applied to laminate. The laminate, bearing a freshly appliedlayer of coating, is then pressed by means of roll 50 into adherentcontact with the heated casting drum 55. The latter should have a smoothhard finishing surface and for this reason polished chromium ispreferred. Nickel and brass are other surfaces which also can beemployed. Roll 66 facilitates stripping of the dried coated laminatefrom the casting drum and roll 68 is simply a windup reel.

The coating mixtures employed for producing the cast surface upon thelaminate are aqueous dispersions of finely divided pigment together witha paper coating adhesive such as casein, soy protein, alpha protein,starch and the like. Such adhesives will normally be water soluble orwater dispersible hydrophilic materials. The synthetic latices ofpolymeric material such as styrene butadiene copolymers, polyvinylacetate and copolymers of a predominate amount of vinyl acetate withother ethylenically unsaturated monomers can be employed as the soleadhesive or in combination with other adhesives. Examples of suitablemineral pigments include calcium carbonate, clay, titanium dioxide, zincoxide, etc. The solids content of the coating will usually be in therange of 25-65% by Weight although it may contain more or less dependingupon the desired thickness of the coating on the laminate. Suchthickness will ordinarily be at least 8 pounds, dry basis, per 3300 sq.ft. of surface. Usually it is desired to work within the range of 4565%by weight solids content in order to give maximum coverage of the sheet.Various additives such as anti-foamers, dispersants, plasticizers, dyes,etc., may also be included in the coating compositions.

The bonding composition for the plies can be identical to the castcoating mixture, as is preferred, or may be of different composition solong as it contains a tacky adhesive. An aqueous dispersion of a vinylacetate polymer without pigment or other adhesive is quite satisfactory.Desirably the bonding composition should have a total solids content of25 to 65% by weight, contain at least 10% by weight of adhesive, and beapplied to the extent of at least 4 pounds, wet basis, per 3300 squarefeet of surface in order to achieve a suflicient degree of bonding andattain the mentioned advantages resulting from the internalplasticization of the laminate with water.

The webs to be simultaneously laminated together and cast coated inaccordance with the invention can be of a wide variety. Thus they can bebleached or unbleached stocks of paper, including paperboard, which havebeen coated, tubsized, or otherwise surface filled. The invention hasparticular application in the lamination of two, or more if desired,individual webs to form a combined laminate of comparatively heavy basisweight, i.e. a caliper of 0.014 inch or more. For this purpose theindividual webs may be identical in terms of formation, composition andthickness or may differ somewhat depending upon the matter of theiravailability and the desired character of the resulting product.

In order to facilitate removal of the cast sheet from the casting drumit is desirable to maintain a monomolecular film of an oleaginousrelease agent on the surface of the drum as disclosed in the US. ReissuePatent No. 23,637 to Montgomery. The film of release agent can be formedby buffing the drum with the oleaginous material and by replenishmentfrom the incorporation of a small amount of the agent directly in theaqueous pigmented coating. The proteinacious cast coatings employed inthe aforesaid Montgomery patent are also suitable for use in the presentinvention.

Although the process has been especially described with reference to theattaining of a cast coated surface by the direct application to thelaminate of coating followed by casting, it will be apparent that thevariations and modifications known in the cast coating art can also beemployed. For example, rather than coating the laminate and casting thatagainst the drum, it is also possible to coat the drum and to press thelaminate into contact with the at least partially dried coating layerupon the drum; this technique being described in US. Patent 2,029,273 toMontgomery. Similarly the use of gelled or plural coatings are alsoknown expedients. As still another means of providing a cast surface, alayer of dried coating as contained on one of the exposed surfaces ofthe laminate may be remoistened with water alone, as by coater 45, andthen pressed into contact with the casting drum as disclosed in BritishPatent 712,717. For that matter the intermediate layer of bondingadhesive can also be a previously dried coating which is simplyremoistened with water, e.g. from coater 25 to thereby unite the plies.

The following examples are given to further illustrate the practice ofthis invention. Proportions of the materials are by weight, unlessotherwise state.

Example I (a) Employing the arrangement illustrated in the drawing, alaminate was formed of two separate moving webs of paper followedimmediately by cast coating.

The cellulosic webs employed had each been formed of fully bleached,sulfate pulped fibers, were uncoated and had a caliper of 0.011 inch.

The same adhesive composition was used to effect both the lamination andcast coating steps. It was prepared of the following ingredients:

parts of finely divided clay 11 parts of casein 10 parts of a polymerfrom 40% by weight butadiene and 60% by weight styrene in the form of a50% total solids latex 0.1 part of formaldehyde, insolubilizing agent0.1 part of monoethanol amine, cooking agent for the casein 0.1 part ofstearic acid, drum release agent made up with water to a total solidscontent of 53.0%, at which it had a pH of 7.0

The coating was applied by a roll and doctor blade coater to theunderside of the top sheet of the two parallel moving webs to the extentof about 4 pounds, dry basis, per 3300 square feet of surface to formthe laminate. The underside of the laminate was then coated with about20 pounds, dry basis, per 3300 square feet with the same composition.The moving coated laminate was pressed into contact with a 12 footdiameter casting drum having a polished chromium finish and which hadbeen internally heated to a temperature of 185 F. It was apparent fromthe operation that the presence of moisture within the laminatefacilitated greater conformity to the casting than that which would beobtained with a single ply of equal thickness without such water beingpresent. When the coating had dried, the resulting sheet materialspontaneously released from the casting drum.

The caliper of the resulting sheet material was 0.0265 inch. It wascharacterized by the typical high gloss finish of cast coated papers.Moreover, it had a flat level apperance with only a very slight degreeof curl, particularly considering the heavy basis weight. There was noobservable tendency of the product to delaminate upon storage or use.

(b) The procedure of (a) was repeated with virtually identical resultsusing paper webs having a caliper of 0.0135 inch. The caliper of theresulting product was 0.0290 inch.

Example 11 The procedure of parts (a) and (b) of Example 1, wererepeated except that an aqueous emulsion of polyvinyl acetate having atotal solids content of was 40 employed as the bonding composition forlaminating the two webs. The properties of the products were nearlyidentical to those of Example I aside from a slightly greater tendencyto curl.

Having described the invention, what is claimed is:

1. Method for the continuous production of cast coated stocks of heavybasis weight comprising the steps of: (a) laminating two moving plies ofpaper by means of an intermediate layer of aqueous based bondingcomposition containing a tacky adhesive, (b) before substantialevaporation of the water contained in the aqueous bonding composition,contacting an exposed surface on one of the plies with an aqueous basedmixture of paper coating adhesive and finely divided mineral pigment,(c) before substantial evaporation of the water contained in the aqueousbonding and coating composition, and drying the thusly laminated andcoated sheet material against a heated finishing casting surface, and(d) finally stripping the resultant sheet material from said castingsurface.

2. The method of claim 1 wherein the said aqueous based bondingcomposition contains a paper coating adhesive and finely divided mineralpigment.

3. The method of claim 1 wherein the said aqueous based bondingcomposition is the same as said aqueous mixture.

4. The method of claim 1 wherein the tacky adhesive of the bondingcomposition is a polymer of vinyl acetate.

References Cited by the Examiner UNITED STATES PATENTS 2,256,916 9/1941Wilshire 156-278 2,361,527 10/1944 Bacon 156272 2,819,184 1/1958 Smithet al 117-64 3,194,706 7/1965 Utschig et a1. 156242 FOREIGN PATENTS469,090 10/ 1935 Great Britain.

EARL M. BERGERT, Primary Examiner.

J. J. BURNS, H. F. EPSTEIN, Assistant Examiners.

1. METHOD FOR THE CONTINUOUS PRODUCTION OF CAST COATED STOCKS OF HEAVYBASIS WEIGHT COMPRISING THE STEPS OF: (A) LAMINATING TWO MOVING PLIES OFPAPER BY MEANS OF AN INTERMEDIATE LAYER OF AQUEOUS BASED BONDINGCOMPOSITION CONTAINING A TACKY ADHESIVE, (B) BEFORE SUBSTANTIALEVAPORATION OF THE WATER CONTAINED IN THE AQUEOUS BONDNG COMPOSITION,CONTACTING AN EXPOSED SURFACE ON ONE OF THE PLIES WITH AN AQUEOUS BASEDMIXTURE OF PAPER COATING ADHESIVE AND FINELY DIVIDED MINERAL PIGMENT,(C) BEFORE SUBSTANTIAL EVAPORATION OF THE WATER CONTAINED IN THE AQUEOUSBONDING AND COATING COMPOSITION, AND DRYING THE THUSLY LAMINATED ANDCOATED SHEET MATERIAL AGAINST A HEATED FINISHING CASTING SURFACE, AND(D) FINALLY STRIPPING THE RESULTANT SHEET MATERIAL FROM SAID CASTINGSURFACE.